A Living Will is a document that alerts health care providers (and your family) what your wishes are about medical treatment and procedures to be performed on you if you are in a terminal and unconscious state. So I assume that you mean a Last Will and Testament, generally called a Will.
Since the childrens' father is alive, he will have rights to the children. If he is truly unsuitable, this may be proven in family court, but if he is truly willing, it will be difficult to overcome. Nevertheless, I would encourage you to write into your Will your wishes as to Guardian of the persons of your minor children, because your Will would be looked at in the event that the father does not wish to take custody of the children or is somehow otherwise prevented from taking custody.
A first step would be to consult with a local estate planning attorney, especially one familiar with local family law. Your childrens' future is too important to not seek the best advice, even if the chance of losing you while they are minors is remote.
But perhaps the best advice I could offer is to stay healthy, because your children need their mother and love her very much.